1. Field of the Invention
The present invention relates to a Corynebacterium microorganism having increased proline dehydrogenase activity compared to its endogenous activity, a method for producing 5′-xanthosine monophosphate or 5′-guanine monophosphate from a culture solution by culturing the transformed microorganism, and use of the microorganism for the production of 5′-xanthosine monophosphate or 5′-guanine monophosphate.
2. Description of the Related Art
5′-Guanine monophosphate (GMP) is a food additive widely used as a flavor enhancer, together with inosine monophosphate (IMP). GMP is known to impart a mushroom-like taste on its own, and to increase the taste intensity of monosodium glutamate (MSG) when combined therewith. It is often used in combination with IMP.
Examples of the methods for the preparation of GMP known thus far comprise (1) the enzymatic degradation of RNA (Ribonucleic Acid) extracted from yeast cells, (2) direct microorganism fermentation to GMP, (3) microorganism fermentation to guanosine, followed by chemical phosphorylation, (4) microorganism fermentation to guanosine, followed by enzymatic phosphorylation, (5) microorganism fermentation to xanthosine 5′-monophosphate (XMP), followed by conversion into GMP by a Corynebacterium strain, and (6) microorganism fermentation to XMP, followed by conversion of XMP into GMP by Escherichia coli. Among them, method (1) has the difficulties of limited material supply and being economically non-beneficial, and method (2) suffers from the disadvantage of being of low yield due to the membrane permeability of GMP. Thus, the other methods are widely used in industrial applications.
In the above described methods, when GMP is produced by conversion of XMP into GMP, it is required to increase XMP productivity or to continuously supply ATP that is used as a cofactor during GMP conversion. To increase XMP productivity, the conventional methods produced guanosine or XMP-resistant microorganisms by mutation. For example, Korean Patent Application No. 10-1991-018061 discloses an XMP aminase-inactive strain capable of producing XMP in high yield, which is semi-auxotrophic for adenine and guanine, tolerant of guanosine analogs and very susceptible to lysozyme, an enzyme which destroys cell walls. Further, Korean Patent Application No. 10-2001-000513 discloses a strain of Corynebacterium ammoniagenes that is able to directly accumulate XMP at high concentration in a culture medium and a method of producing XMP using the same, in which the strain is prepared by irradiating the mother microorganism with UV light, treating with the mutagen N-methyl-N′-nitro-N-nitrosoguanidine (NTG), and selecting a mutant tolerant of norvaline, an analog of valine which affects the biosynthesis of XMP. Furthermore, Korean Patent Application No. 10-2008-006537 describes a method of increasing XMP yield in which purN and purH genes involved in the biosynthesis of XMP are modified.
As mentioned above, for the conversion of XMP into GMP, it is critical to supply ATP which is used as a cofactor during GMP conversion. Most of the ATP used in the conversion of XMP to GMP is supplied from an XMP-producing strain. In the conversion approach, xylene increases the membrane permeability of ATP and XMP, and addition of xylene to the medium allows ATP and XMP to penetrate into a GMP-producing strain, followed by the conversion of XMP into GMP. Therefore, the approach to GMP production takes the strategy of increasing ATP productivity.
The conversion from XMP into GMP is represented by the following reaction formula:

That is, a continuous supply of ATP, serving as a cofactor, is essential for the conversion process in which XMP is primarily produced and then converted into GMP by addition of an enzyme or microorganism having XMP aminase activity to the culture medium comprising XMP and a microorganism. Thus, it is very important to enhance ATP productivity of the XMP-producing strain. The AMP produced in the conversion process is reused as a substrate for ATP production. In fact, adenine-based nucleotides are recycled for the production of ATP in the conversion process.
Hence, improvement of XMP productivity is necessary for the high production yield of GMP, and XMP productivity can be improved by increasing ATP productivity. Based on this background, the present inventors have made many efforts to develop a method for increasing ATP productivity. As a result, they found that enhanced activity of proline dehydrogenase improves production yields of XMP and GMP by increasing ATP production.
Therefore, the present inventors identified a gene responsible for the improvement of XMP production yield, designed a vector comprising the gene, and prepared a microorganism of the genus Corynebacterium transformed with the vector, and they found that XMP or GMP can be produced from the microorganism in a high yield, thereby completing the present invention.